Some Experimental and Theoretical Observations Concerning Mass Flow in the Vascular Bundles of Heracleum

The theory and practice of applying the thermodynamics of irreversibleprocesses to mass-flow theories is presented. Onsager coefficientswere measured on cut and uncut phloem and cut xylem strandsof Heracleum muntegazzimum. In 0.3 N sucrose + 1 mN KC1 theyare as follows. In phloem, L_EE = 5 ? 10–⁴ mho cm^–1,L_pE = 9 ? 10^–6 cm³ s^–1 cm^–2 volt^–1 cm,and L_PP = 0.16 cm³ s^–1 cm^–2 (J cm^–3)^–1cm. In uncut phloem strands L_EE is about 1 ? 10^–3 mhocm^–1. In xylem in 2 x 10^–3 N KCI, L_pp = 50 to 225,L_PE = 2 ? 10^–4, and L^EE = 4 ? 10^–3. The measurementsare tentative since the blockage of the sieve plates is an interferingfactor, but if they are valid they lead to the conclusion thatneither a pressure-flow nor an electro-kinetic mechanism envisaginga ‘long distance’ current pathway can be the majormotive ‘force’ for transport in mature phloem. Measurementsof biopotentials along conducting but laterally detached phloembundles of Heracleum suggest, nevertheless, that there may bea small electro-osmotic component of at least 0.1 mV cm^–1endogenous in the phloem.